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Isolators are used to support and isolate external vibrations, or support a vibrating load and prevent its energy from disturbing the surroundings.
There are two major lines of isolators in the Firestone range - Firestone Airmount® isolators and Firestone MarshMellow® springs. Each have their distinct benefits over other typical industrial vibration isolators, and each has advantages over the other. Many industrial vibration isolation problems can be solved using either line, and a complete analysis of potential solutions to these problems should include investigating options using both lines of isolators.
While it may be theoretically possible to construct a solid isolator that has as low a spring rate as an air spring, it is often impossible practically. Air springs can be constructed to have spring rates as low as a coil spring with over 9 inches [299 mm] of static deflection. This would necessitate a coil spring with an impossibly long free height.
Installed heights for air springs are normally less than an equivalent coil spring. This gives the designer greater freedom to control the height of his equipment.
Air springs also have a unique capability unavailable in a solid spring: they are adjustable. While it may be theoretically possible to have a coil spring as effective as an air spring at a particular load, it is impossible to construct a spring that has as wide a load range, or is as effective throughout the range. An air spring can support loads varying by hundreds or even thousands of pounds with the same effective isolation. For the designer this gives a generous tolerance to his calculations. An additional benefit is the ability to maintain a level surface with a compliant isolation system. As a load shifts, air springs can be inflated or deflated to maintain a constant height, and therefore a level surface.
Most solid forms of isolation devices have historically been used with large inertia masses whose function is to reduce the amplitude of the vibration to within the ability of the isolator to comply to the motion. This allows a relatively stiff isolator to do an acceptable job, and a more compliant one to do a good job. However, this is expensive, and is sometimes impossible without serious structural enhancement. With an air spring, you have a more compliant support. The low natural frequencies and large travel of the air spring allow it to reduce the need for a large inertia mass.
Like Marsh Mellow® springs, Airmount® isolators have a variable spring rate. The variable spring rate is due to air spring geometry, internal air pressure, and changing effective area. The effect of this variable spring rate is that an Airmount® isolator can perform with constant efficiency through a wide range of loads. It also means that a single style of air spring can be used at every mounting point of machinery or systems that are unevenly loaded without sacrificing this efficiency at any of the mounting points. Traditional constant spring rate isolators such as coil springs do not have this ability to operate with constant efficiency through a wide range of loads. In fact, a coil spring designed to operate at a given height loses isolation efficiency with even very small decreases in the supported load.
Given that a lower system natural frequency provides superior isolation and that less variability in natural frequency provides more constant isolation, the benefits of the Airmount® are clear.
For example, in an application designed to use either coil springs or air springs to support loads ranging from 1500 lbs [6.67 kN] to 3000 lbs [13.3 kN], a system using a coil spring 8.5 inches [216 mm] tall with a constant spring rate of 1200 lbs per inch [210 kN per meter] would have a natural frequency ranging from 118.9 cpm at 3000 lbs [13.3 kN] to 171.6 cpm at 1500 lbs [6.67 kN]. For the same loading, a system using a style 20-2 Airmount® isolator would have a natural frequency with a range from 95 cpm at 3000 lbs [13.3 kN] to 103 cpm at 1500 lbs [6.67 kN].
Airmount® isolators can provide the highest degree of isolation of any type vibration isolator. System natural frequencies as low as 60 cpm (1 Hz) are available. The addition of an auxiliary reservoir can provide even lower system frequencies. In order to achieve similar results from a conventional coil spring isolator, a real static deflection of 9 inches (229 mm) would be required. This would require a very long free length for a coil spring, and be very difficult to make stable.
Airmount® isolators are unique in that the system's natural frequency does not change significantly with changes in load at pressures greater than 40 psig (2.8 bar). This unique feature, combined with accurate height control, will allow the use of the same Airmount® isolator at each mounting point of an unevenly loaded machine.